A re-configurable spatial light modulator (SLM) based on liquid crystal as well as other types of devices are widely used for controlling and manipulating optical beams. In diffractive mode they may be used for three dimensional (3D) imaging. 3D imaging is described further in U.S. Pat. No. 6,437,919 the specification of which is herein incorporated by reference.
The SLM modulates the complex amplitude of an incoming wave front (i.e. changes its phase and/or amplitude), which causes it to propagate in the desired manner. The SLM generally comprises a liquid crystal panel containing a number of individually addressed pixels, onto which a diffraction pattern or Computer Generated Hologram (CGH) is written.
CGH 3D display systems may use a computer to generate and/or store electronic copies of the hologram. This hologram is then replayed on an SLM which is switched to modulate (in transmission or reflection) light from a source which then passes through suitable replay optics, thereby providing a visible three-dimensional image to observers.
A single EASLM may be addressed to produce successive different images, which are imaged sequentially onto an OASLM arranged in a matrix of segments, which forms a complete display. Once all the component images have been written to the OASLM a complete image or pattern can be presented to an observer, e.g. by illumination of the whole OASLM matrix by laser light. This system may be referred to as Active Tiling™, and is described further in U.S. Pat. Nos. 6,437,919 and 6,654,156, the specifications of which are herein incorporated by reference.
The SLM system may include a layer of liquid crystal material arranged between two electrode-bearing walls to form a liquid crystal cell. The liquid crystal material is switched by application of electric waveforms to the electrodes. A characteristic of liquid crystal materials is that they deteriorate under the effects of long-term direct current (DC) voltages. The SLM system is designed so that the liquid crystal material is maintained under a net zero DC voltage and so that drive schemes for addressing the SLM system results in DC balance. A net zero voltage may be maintained over a reasonable time period of several seconds.
The EASLM may comprise a liquid crystal cell formed by two walls enclosing a layer of the smectic liquid crystal material. Transparent electrode structures are formed as strips of row electrodes on one wall and strips of column electrodes on the other wall. Electrode intersections define pixels where the optical state of the liquid crystal material is switched by application of an electric voltage to appropriate row and column electrodes. The electrodes receive electrical signals from driver circuits controlled by a display controller. The EASLM may use an integrated circuit backplane. A DC balance is achieved by addressing the SLM system to form a positive image followed by addressing the SLM system to form the inverse or negative image.
An OASLM is basically similar to the EASLM but may include a layer of photosensitive material located between electrodes on one wall and the ferroelectric liquid crystal material. The electrodes may be segmented so that electrical contact is made separately to each segment. An image may be applied to more than one segment (and in some cases to all of the segments) but a voltage is only applied to one segment to effect latching of the image only at that one segment. The OASLM is addressed by an application of a voltage to the electrodes and a simultaneous application of light to selected parts of the photosensitive material. This combination of applied voltage and applied light causes the liquid crystal material to switch at illuminated parts while non-illuminated parts remain unswitched. A display generated by the SLM system may be viewable from the side of the OASLM that is remote from the photosensitive layer.
Drive schemes may be used to provide DC balance are described further in “Optimisation of ferroelectric liquid crystal optically addressed spatial light modulator performance”, F. Perennes & W. A. Crossland, Opt. Eng. 36 (8) 2294-2301 (August 1997); Applied Optics Vol. 31, No. 32, pp. 6859-6868, 10 Nov. 1992. The operating theory of spatial light modulators is described farther in “Spatial Light Modulator Technology, Materials, Devices and Applications”, edited by U. Efron, published by Marcel Dekker Inc. 1995.
In the SLM system described above, a pattern of light may be transmitted through or reflected from the EASLM, on to each segment of the OASLM in turn. Alternatively, the several images may be applied to all segments of the OASLM. For each time period in which an image is loaded into the EASLM, played onto the OASLM, and latched into the OASLM, there follows an equal time period in which an inverted image is loaded into the EASLM and held in order to maintain DC balance at the EASLM. This time period is wasted from the point of view of the OASLM device. It does not contribute to improving the OASLM image.